1. Field of the Invention
The present invention relates to an arrangement of controlling digital data transfer between digital circuit packages or the like having different bus architectures. More specifically, the present invention relates to such an arrangement by which different architecture packages are able to implement data exchange without any changes of the bus architectures thereof.
2. Description of the Prior Art
In a microcomputer system, it is often the case that a central processing unit (CPU) originally installed is required to be replaced with a more effective CPU having a wider data bus. By way of example, an 8-bit data bus type CPU is required to be replaced with a CPU having 16-bit data bus. In such a case, a package including a new 16-bit CPU is no longer compatible with circuitry installed in the package itself and also incompatible with external packages.
In order to adapt the new CPU to the requirements of the circuits within the CPU package and also render the CPU package compatible with external packages, the following approaches have been proposed.
A first conventional approach is to select a CPU which has wider internal buses (16-bit for example) and external buses each equal to an original narrow bus (8-bit for example). Accordingly, the CPU newly installed becomes compatible with blocks coupled to the CPU. However, data transfer between the new CPU and storage means such as read-only-memories (ROMs) and/or random-access-memories (RAMs) within the same CPU package are implemented by way of the previous narrow buses, and hence an overall processing speed can be raised merely by increase in a new CPU clock speed.
Another conventional approach to solving the above-mentioned difficulties is that the least significant bit line A0 of a newly installed CPU is rendered inoperative and hence data transfer with external packages are performed using every other address (practically even addresses). It follows that this prior art has a disadvantage that only one half of the address area of an external package can be accessed.